Unusual trackways left by some of the world’s largest-ever dinosaurs have left paleontologists stymied by what they seem to suggest. Giant round footprints found in Texas clearly show that huge four-footed sauropods once roamed the land here, and yet, they’ve only found sets of prints from the two front feet.

Unless sauropods were prone to doing handstands, scientists have another suggestion for why this may be so: the dinosaurs may have waded across a lake or river, using just their front feet to punt along the bottom.

In a new study published in Ichnos, researchers from the Heritage Museum of the Texas Hill Country, the Houston Museum of Natural Sciences, and Purdue University hypothesize about what scenario could lead these prehistoric giants that needed four immensely strong legs to hold their massive weight to have been possibly bi-pedal.

The footprints – some measuring 70 centimeters (27.5 inches) across – were found in a limestone quarry near Austin back in 2007. The layer of rock they are in dates to around 110 million years old. It’s difficult to estimate a dinosaur's size or species just by footprints, but the size of these suggests it could be one of the three largest sauropods found in this part of America: Astrophocaudia, Cedarosaurus, and Sauroposeidon, the tallest dinosaur ever.

The idea that these massive creatures walked on two legs, and those being the front legs rather than the hind, is absurd, experts say. However, to find manus-only (meaning just the forelimbs) tracks of large sauropods is not unheard of, though it is rare. The theory that they may have been created by a dinosaur “swimming” goes back to 1940, when similar manus-only prints were found in an ancient river bed in Texas, back when sauropods were thought to be aquatic animals.

However, the more dinosaurs we dug up, the more we understood they were terrestrial creatures, and manus-only footprints were probably to do with weight distribution. One of the prevalent theories was that the center of mass for these large dinosaurs was toward their front, meaning their front feet would bear the load, creating deeper impressions in the ground.

However, other sauropod footprints across Texas show all four feet, meaning the hindfeet left impressions just as deep as the front ones. These manus-only footprints were also oddly splayed at a distance further than other finds. This suggests that the bi-pedal behavior was perhaps due to "unusual locomotion" by these animals, which led the researchers back to the "swimming" sauropods.

You might think it's inconceivable that a terrestrial creature that weighed as much as 60 tons could swim, but these dinosaurs had huge lungs and air sacs in their bones, which would make their bodies oddly light. However, the researchers hypothesize that the makers of these tracks were not quite swimming, but wading through water that had been around shoulder height, using their front limbs to punt themselves along the bottom to maintain control and balance. 

"Although hypothesized unusual behaviour would not necessarily involve ‘swimming’, it is worth considering the possibility that... manus-only sauropod trackways were made by dinosaurs that were wading in water deep enough for their makers to punt, pulling themselves along by their forefeet, while their hind legs floated above the bottom," they write.

"Such a punting sauropod might then be able to glide with its body supported by the water, thereby taking longer oblique paces and creating a wider trackway... If much of the dinosaur’s weight was buoyed up by the water, this might also result in shallowly impressed manus prints."

The researchers note that until there is more evidence, either from remains and other footprints discovered in this part of Texas, or other manus-only trackways in areas that were conclusively once water-logged, this explanation can only remain a hypothesis, but we are with lead author Dr James Farlow when he told New Scientist: "[I]f you ask me in my heart of hearts what I would like to be true, I’d like them to be punting.”